Rat Trap Bond Construction, Benefits, and Drawbacks

 Rat Trap Bond Construction, Benefits, and Drawbacks

Rat trap bond is a type of modular masonry bond in which the bricks are placed vertically, creating a cavity in the wall while maintaining the same wall thickness as a conventional brick masonry wall. It is also referred to as a Chinese brick bond.

Because of the cavity formed in the wall, the purpose of using this type of masonry bond is to reduce the number of bricks and mortar required when compared to the English/Flemish bond.

Architect Laurie Baker pioneered it in Kerala in the 1970s, where it was widely used for its lower construction costs, reduced material requirements, and improved thermal efficiency over conventional masonry walls without compromising wall strength.

This article discusses the material criteria, construction, benefits, and drawbacks of rat trap bond masonry.

Choice of Bricks

Because there are fewer bricks used in the construction of rat trap masonry, the criteria for selecting them are critical.

1. The size of the bricks used must be uniform, and variations in size are not permitted. Brick sizes that are acceptable in Indian scenarios are: length 220-250 mm, width 100-115 mm, and height 65-75 mm.
2. The bricks' edges and corners must be straight and sharp, as well as perfectly rectangular in size.
3. Because the masonry is modular, having uniform brick sizes is critical for achieving good strength and finish.

Table 1 shows the required material strength. Bonded Rat Trap

Type of Construction	Recommended Compressive Strength of Bricks Best Practice Minimum Allowable	Recommended Mortar Ratio
Load bearing, Double storied	40 - 50 kg/cm2	1:5
Load bearing, Single storied	35 - 40 kg/ cm2	1:4
Infill masonry in frame structure, no restriction on number of floors	Min 35 kg/ cm2	Not less than 1:4

Rat Trap Bond Construction

1. The bricks are stacked vertically so that the 110 mm face is visible from the front elevation rather than the 75 mm face (considering brick of standard size 230 X 110 X 75 mm).
2. Because the wall width is kept at 230mm, a cavity is created inside the wall.
3. However, the first and last layers of the masonry are built using conventional sold masonry.
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5. For fixing frames, the sill, lintel, and sides of openings are made of solid masonry (no cavity).
6. Vertical and horizontal reinforcement bars are installed in the cavities to strengthen the masonry.
7. Electrical conduits and plumbing pipes can be placed inside the cavity with advance planning for improved aesthetics.

Rat Trap Bond Benefits

1. The masonry cavities serve as thermal insulators. As a result, the interiors remain cooler in the summer and warmer in the winter.
2. When compared to traditional brick masonry, Rat Trap masonry uses fewer bricks and mortar, lowering the cost of masonry by up to 30%.
3. The number of bricks used in rat trap masonry is 470, whereas it is 550 in conventional masonry.
4. Rat trap masonry walls can be used as load-bearing walls as well as thick partition walls.
5. When the rat-trap bond is left exposed, it creates an aesthetically pleasing wall surface and saves money on plastering and painting.
6. Because this type of masonry contains 30% cavities, the dead load of the structure is reduced, reducing the structure's supporting members such as columns and footings.
7. Reinforcement bars can be inserted through the cavity until the foundation for added structural safety.
8. Many buildings built decades ago have demonstrated that this type of walling technology is long-lasting and low-maintenance.

THE CRITERIA FOR BRICKS SELECTION

• Although regional variations in brick size are unavoidable, the acceptable sizes (in the Indian context) are as follows: Length 220-250 mm, Width 100-115inm, and Height 65-75mm.

• The brick should be rectangular with straight and sharp edges, and the brick size should be uniform so that the brickwork can be designed and constructed in a modular pattern with good strength and finish.

The cement-sand proportion in mortar is determined by the quality of the brick and the building design, which determines the load on the masonry. As a general rule, a 1:5 (cement: sand) mortar is recommended for compressive strength of bricks greater than 50kg/cm2, and a 1:4 mortar for strengths between 35 and 50 kg/cm2. Although a thickness of 1/2" is recommended for mortar joints, a slight reduction in the thickness of vertical joints is permissible because the contact area at the side of the brick is much smaller than at the bottom and top. It is critical to ensure that the entire vertical joint is filled with mortar; otherwise, the brick masonry wall will leak.

According to conventional masonry design principles, corners and openings are weak points in masonry and should be designed for adequate strength, depending on factors such as building design and seismic strengthening requirements. As a result, all masonry corners and opening ends in rat-trap brickwork are solid, with no cavities.

MODULAR DRAWING:

To take full advantage of the technique, the masonry should be designed in a modular pattern during the design stage, after the prevailing brick size available for use has been determined. Half bricks/quarter bricks should not be used in rat-trap brickwork, contrary to their common use in conventional brickwork. This will disrupt the staggering of joints in rat-trap brickwork and compromise its integrity.